1. Field of the Invention
This invention relates in general to circuit-interrupters and more particularly to fluid-blast circuit-interrupters of the puffer-type.
2. Description of the Prior Art
The advantages of using sulfur-hexafluoride (SF.sub.6) gas in fluid-blast circuit-interrupters are well known to those skilled in the art. There are two basic types of fluid-blast circuit-interrupters using SF.sub.6 gas: two-pressure interrupters and puffer-type interrupters. The two-pressure interrupters uses a gas compressor to produce a reservoir of high-pressure gas, which creates a blast of gas to extinguish the arc established between separating contacts. Since the gas storage reservoir may be large, and the gas pressure inside it high, this type of breaker is suitable for higher interruption ratings. The puffer interrupter, on the other hand, maintains a relatively-low ambient gas pressure inside the interrupter, typically about 60 p.s.i., for example, and produces a gas blast for the purpose of arc extinction by means of a transient compression of gas performed by a movable piston member. The puffer is normally used for lower interruption ratings only. The prime advantage of a puffer interrupter is its lower cost, for it does not require heaters to prevent gas liquification of the gas or expensive compressor components, which are necessary in a two-pressure circuit-breaker. Therefore, it would be desirable to use a puffer-type interrupter in service categories requiring a higher interruption rating.
The size and cost of a circuit-interrupter actuating mechanism can be minimized when interrupting capability is limited to the service rating, plus a sufficient safety margin. One method for varying the interruption capability requiring few component modifications is to vary the degree of compression to which the arc-extinguishing fluid is subjected prior to initiation of the arc-extinguishing blast. However, varying the degree of gas compression in previous circuit-interrupters has often required a delay in the separation of contacts resulting in a delay in arc establishment. It would be desirable to produce a circuit-breaker design suitable for a variety of ratings by varying the degree of gas compression without delaying the moment of arc initiation.
In U.S. Pat. No. 3,331,935, entitled "Gas-Blast Circuit-Breaker Having Dual Piston Means Providing Double-Acting Puffer Arrangement," issued July 18, 1967 to Stanislaw A. Milianowicz, and assigned to the assignee of the instant application, there is disclosed a gas-type circuit-interrupter using two pistons to compress arc-extinguishing fluid within the same volume, thereby producing two blasts of fluid. It would be desirable to produce a circuit-interrupter generating two blasts of arc-extinguishing fluid with a simpler mechanism.